I plan to SLI two GTX580 Lightning Xtreme Gpus on the Gigabyte Z77X-UD3H Mobo.
As for the placement of the cards, I am unsure of. Would I put one in the x16 slot and the other in the x8 or would both slots need to be x16?

You'll put one in the x16 slot and the other in the x8 slot. This will drop the x16 slot down to x8 speeds on your motherboard, so both cards will run at x8. Since they're PCI-E 3.0 slots though, it shouldn't be an issue.

Dang, Raptor is faster.

Edit: Never mind, the cards need to be PCI-E 3.0 to run at the faster speeds, so they'll both be at 8x PCI-E 2.0.

You'll put one in the x16 slot and the other in the x8 slot. This will drop the x16 slot down to x8 speeds on your motherboard, so both cards will run at x8. Since they're PCI-E 3.0 slots though, it shouldn't be an issue.

Dang, Raptor is faster.

Edit: Never mind, the cards need to be PCI-E 3.0 to run at the faster speeds, so they'll both be at 8x PCI-E 2.0.